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Bob McDavitt's ideas for sailing weather around the South pacific

27 September 2009

BOBGRAM7 issued 25 Sep 2009

WEATHERGRAM
YOTREPS
Issued 27 September 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Well, the days are now longer than the nights and soon the annual
migration from tropics to NZ/OZ will be upon us, analysis paralysis will
be returning to the Minerva virtual yacht club. At this stage you've
probably heard that El Nino is in the equatorial Pacific Ocean, well
that's true but it isn't yet manifest in the atmospheric weather
patterns. This is expected to gradually become noticeable over the next
few months, as an equator-wards shift of our favourite weather zone: The
SPCZ, the subtropical ridge and the roaring 40s.
Since this is the season when the subtropical ridge normally shifts
southwards, maybe the resultant kerfuffle will appear to be just a
continuation of the current pattern.

Anyway those of you heading for NZ or Aus in yachts that average around
150 mpd or less are reminded that fronts and troughs cross the Tasman
Sea /NZ area around once every 4 days or so, thus your voyage is likely
to encounter a front. Normally the bets latitude to do this is around
30S--- sort of a mid-way rite of passage, and at that latitude fronts
are normally at their weakest, plus, if you time in rightly, it will
allow you to make landfall "between fronts" and that's a good idea.

TROPICS/SUBTROPICS
Cyclonic conditions are raging in Philippines and NW Pacific at present
and this extra convection extends across the equator to just north of
Solomons to Tuvalu, the weakens a little in a South Pacific Convergence
zone this stretches to between Southern Cooks and French Polynesia FP .
SPCZ is expected to take a rare excursion northwards across FP this week
- allowing, after its passage, a nice voyage westwards from Tahiti.

The northern end of a elderly mid-latitude front dumped hail into Suva
market last Tuesday-just in passing-just some cold air getting from
mid-latitudes into the tropics.

SUBTROPICAL RIDGE ZONE
Not much action expected here this week.
Today's HIGH in North Tasman Sea/New Caledonia area is expected to
weaken but remain as a zone of light winds that will migrate east along
around 20 to 30 S reaching Niue around 03 Oct UTC.

Another HIGH is expected to form in North Tasman Sea between Lord Howe
Island and 25S on 30 Oct and reach to 180 line around 4 Oct UTC. There
should be a squash zone of enhanced easterly winds along 10 to 20S to
north of this HIGH.

TASMAN SEA/ NZ AREA
Basically it is expected to be a week of two LOWS/

Low 1 is already deepening off Sydney, with strong SW winds over New
South Wales. Frontal zones associated with this low should cross NZ on
Monday, and the LOW itself, with attendant intense showers in a cols
pool, is expected to cross NZ on Wed, followed by strong westerly winds
on Thu. Avoid. Should be OK to make landfall in Northland on
Thursday-Friday-Saturday, but please check.

Second LOW is expected to form off southern New South Welsh coast on
Friday and turn into a trough as it crosses South Island on Saturday 2
Oct and North Island on Sun 4 October. Avoid, and this timing may
change so update.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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